Pour lukewarm milk and water into a bowl. Sprinkle sugar and yeast on top. Stir once or twice. Set aside for at least 5 minutes until next step calls for yeast.
Sift wheat flour and bread flour and salt into a large bowl. Using a wooden spoon, mix butter and mashed potatoes into flour. Mixture will be very stiff. Pour yeast mixture into flour mixture and mix with a wooden spoon.
Place dough on a lightly floured surface and knead for 8 to 10 minutes until dough becomes smooth and elastic. Place dough in a greased bowl, roll dough in buttered bowl, and turn so top is oiled.
Cover with plastic wrap and set aside in a warm area until it is double in size. Punch dough down and place into a greased bread pan. Cover and leave in a warm place until dough is level with top of pan. Bake at 400 degrees for 40 to 45 minutes or until nicely browned on top and loaf sounds hollow when tapped. Cool on a wire rack.